Invalid returned from hospital lunch (4)
I believe the answer is:
null
'invalid' is the definition.
(similar in meaning)
'returned from hospital lunch' is the wordplay.
'returned' shows that the letters should be reversed in order.
'from' indicates a hidden word.
'llun' is hidden within 'hospital lunch'.
'llun' back-to-front is 'NULL'.
(Other definitions for null that I've seen before include "Of no effect, value" , "Without legal force" , "Without value or consequence" , "Lacking any legally binding force" , "... and void, lacking any legal or binding force" .)